Blood vessel occlusions are commonly treated by mechanically enhancing blood flow in the affected vessels, such as by employing a stent. Stents act as scaffoldings, functioning to physically hold open and, if desired, to expand the wall of affected vessels. Typically stents are capable of being compressed, so that they can be inserted through small lumens via catheters, and then expanded to larger diameters once they are at the desired location. Examples of stents disclosed in the patent literature include U.S. Pat. No. 4,733,665 (Palmaz), U.S. Pat. No. 4,800,882 (Gianturco), U.S. Pat. No. 4,886,062 (Wiktor), U.S. Pat. No. 5,061,275 (Wallstein), and U.S. Pat. No. 6,605,110 (Harrison), and U.S. 2003/0139800 (Campbell).
FIG. 1 illustrates a conventional stent 100 formed from a plurality of struts 104. The struts 104 are radially expandable and interconnected by connecting elements or links 108 that are disposed between adjacent struts 104, leaving lateral openings or gaps 110 between adjacent struts 104. The struts 104 and the connecting elements 108 define a tubular stent body 112 having an outer, tissue-contacting abluminal surface and an inner, blood flow contacting luminal surface.
Stents are used not only for mechanical intervention but also as vehicles for providing biological therapy. Biological therapy can be achieved by medicating the stents. Medicated stents provide for the local administration of a therapeutic substance at the diseased site. Local delivery of a therapeutic substance is a preferred method of treatment because the substance is concentrated at a specific site and thus smaller total levels of medication can be administered than with systemic dosages that often produce adverse or even toxic side effects for the patient.
One method of medicating a stent uses a polymeric carrier coated onto the surface of the stent. A composition including a solvent, a polymer dissolved in the solvent, and a therapeutic substance dispersed in the blend is applied to the stent by immersing the stent in the composition or by spraying the composition onto the stent. The solvent is allowed to evaporate, leaving on the stent strut surfaces a coating of the polymer and the therapeutic substance impregnated in the polymer. Other known drug deposition methods include roll-coating, electrostatic spraying, and vapor deposition.